1. Field of the Invention
The present invention relates to a semiconductor device employing a Darlington transistor used within an ignition device for an automobile.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional semiconductor device including a Darlington transistor for use within an ignition device for an automobile. In the semiconductor device, a Darlington power transistor 100 is constituted by NPN transistors 1 and 2. The transistor 1 has a collector connected to a first output terminal 6 and an emitter connected to a second output terminal 7. The transistor 2 has a collector connected to the first output terminal 6, an emitter connected to a base of the transistor 1 and a base connected to an input terminal 5.
Also connected to the first output terminal 6 is one end of an ignition coil 200, the another end of the ignition coil 200 being connected to an ignition plug (not shown) mounted within an engine room. Between the base and the emitter of the transistor 1 is interposed a resistor 3. Likewise, a resistor 4 is inserted between the base and the emitter of the transistor 2. A control circuit 300 is for generating a pulse signal 20 in response to a signal received at a signal input terminal 30, the pulse signal 20 being to be given to the input terminal 5. Another resistor 10 is interposed between the second output terminal 7 and a ground to restrict current flow into the transistor 1.
An avalanche diode 8, with a cathode connected to the first output terminal 6 and an anode connected to the base of the transistor 2, protects the Darlington transistor 100 against breakage if high voltage is impressed on the first output terminal 6.
Now, operations of the conventional semiconductor device will be explained. Provided with a signal received at the signal input terminal 30, the control circuit 300 generates a pulse signal 20 for turning the Darlington transistor 100 on and off. In response to the pulse signal 20 given at the input terminal 5, the Darlington transistor 100 turns on or off. Shift in the Darlington transistor 100 from conductive state to non-conductive state gives rise to high voltage at the ignition coil 200. Supplied with the generated high voltage, the ignition plug (not shown) installed in the engine room sparkles, whereby fuel in an engine explodes and the engine starts up.
The high voltage generated at the ignition coil 200 is impressed on the first output terminal 6 as well, so that the avalanche diode 8 turns on. Current thus allowed to the avalanche diode 8 flows into the transistor 2 where it becomes base current which is amplified thereat. The current amplified at the transistor 2 is next given to the base of the transistor 1. The transistor 1 further amplifies the current so that current equivalent to the current amplified at the transistor 1 is pulled out from current flowing in the first output terminal 6. As a result, the first output terminal 6 is freed from high voltage impressed thereupon, which in turn prevents the Darlington transistor 100 from breakage. Thus, owing to the structure that current flowing in the avalanche diode 8 is amplified at the transistors 1 and 2, the conventional semiconductor device is advantageous in that reduction of the avalanche diode 8 in size is possible.
As heretofore described, the conventional semicondutor device for an ignition device protects the Darlington transistor 100 against breakage by means of the avalanche diode 8. However, this approach has a problem. If the avalanche diode 8 is formed by a collector-base junction of a transistor, high avalanche voltage would result at the avalanche diode 8, whereby the avalanche diode 8 would not be able to play its role of protecting the Darlington transistor 100 against breakage. Hence, in order to obtain an avalanche diode 8 at which avalanche voltage is low, fabrication of the conventional semiconductor device must include one more diffusion step in addition to a diffusion step for forming the transistors 1 and 2. Thus, the semiconductor device becomes expensive.